The present invention relates to the field of vacuum switches for the low-voltage range, and especially low-voltage contactors comprising a vacuum switching tube having a vacuum switching chamber and a first contact fixedly arranged therein, as well as a lead-in pin movable relative thereto having a second contact, and wherein the vacuum switching tube has an insulating path or member which is surrounded on the vacuum side, at least partially, by shielding.
Every switch contains a high-resistance insulating path which must not change, as far as possible, during the entire life of the switch. In vacuum switches, dust and water vapor and the like can precipitate on the insulator on the side exposed to the atmosphere and, on the other hand, metal vapor from the evaporating contact material can precipitate on the vacuum-side surface, and thereby adversely affect the insulating capability of the insulator. Therefore, shielding on the vacuum side of the insulators is required in order to prevent metal vapor condensation. Depending on the voltage applied, such shielding can in part be rather expensive due to the mechanical design.
In low-voltage switches only relatively short insulating paths are required for the potential isolation because of the low nominal voltage. In practice, therefore, insulator shapes also are used besides coaxial insulators, which have a radial insulating path. A low-voltage vacuum switch is described, for instance, in DE-AS 19 57 829 in which a base plate of the switching tube is designed as an insulator and on the vacuum side of which is provided a lid-shaped conical shield. From US-LP 4077 M4, a vacuum switch is further known in which the insulator, as part of the switching chamber, forms the entire hollow cylinder, and the shielding is formed by a concentric ring disposed therein. The inner ring, which has a length smaller than the insulator, substantially encloses the contact region including the rigid contact and the contact pin and attached contact movable relative to the rigid contact. Other vacuum switches also include shielding constructed from several individual parts.
It is desired, for cost reasons, to make the insulating path as short as possible. In contrast with the described state of the art, in the present invention only a part of the switching tube is designed as ceramic, for which purpose substantially ring-shaped elements can be used. Even if short insulators are electrically sufficient for the low-voltage range, certain requirements with respect to the shielding must nevertheless be met. For example, it must be taken into consideration that, especially if switching tubes are used as contactors for the low-voltage range, particularly large numbers of switching operations of, for instance, several million switching cycles, are required. With the shielding customary to date, some metal vapor molecules can, in the course of time, migrate to the insulator by reflection, condense there and thus form an electrically conducting coating.